1. Field of the Invention
The present invention relates to a digital filter and a vehicle driving force control apparatus configured to reduce a vehicle body vibration through use of the digital filter.
2. Description of the Related Art
Hitherto, there is known a technology of controlling a driving force of a driving device, e.g., an engine, to thereby reduce a vehicle body vibration. For example, in order to reduce the vehicle body vibration, an apparatus proposed in Japanese Patent Application Laid-Open No. 2007-237879 is configured to use a notch filter to filter a signal representing a driver required driving force, and control a driving force of a driving device based on the driver required driving force after the filtering. In this apparatus, a notch frequency of the notch filter is set to a value for reducing a frequency component of a pitching vibration of the vehicle body. As a result, the pitching vibration of the vehicle body can be reduced. As such a notch filter, a digital filter is used.
An output signal value after a specific frequency component is removed by the digital filter in this way can be used to control various devices. However, the output signal value after the filtering by the digital filter has such a characteristic as to be delayed from an input signal value before the filtering, and not to monotonically follow a change in the input signal value (that is, to pulsate).
Therefore, when an operation of a device is controlled based on the output signal value, the operation of the device may become unstable. Moreover, a fluctuation range of the output signal value may exceed a fluctuation range of the input signal value, that is, the output signal value may overshoot or undershoot with respect to the input signal value.
For example, a case is considered in which a control system for a device is constructed to start an operation of the device when the output signal value of the digital filter becomes more than a first threshold value, and to stop the operation of the device when the output signal value becomes less than a second threshold value (<first threshold value). On this occasion, a range of from the second threshold value to the first threshold value is a dead zone for preventing hunting.
When the input signal value input to the digital filter increases to become more than the first threshold value, the output signal value increases accordingly to become more than the first threshold value. Then, even when the input signal value is maintained to be a value larger than the first threshold value, the output signal value after the filtering may temporarily increase/decrease to become less than the second threshold value. In this case, the device that needs to be maintained in operating condition temporarily stops.
A description is now given of the problem in one example. For example, a case is considered in which, in a hybrid vehicle in which a hybrid system constructed by combining an engine and a motor/generator is used to generate a driving force, the driving force is controlled by using an output signal value of a notch filter (digital filter) to decrease a pitching vibration of a vehicle body. In this hybrid vehicle, a signal representing the driver required driving force determined by an accelerator operation amount and a vehicle speed is input to the notch filter, and an output signal value of the notch filter is used to control the driving force of the hybrid system. In this apparatus, a notch frequency of the notch filter is set to a value for reducing a frequency component of the pitching vibration of the vehicle body. Thus, the pitching vibration of the vehicle body can be suppressed.
In the hybrid vehicle, operations of the engine and the motor/generator are switched depending on the value of the driver required driving force. For example, such a control system is considered that when the driver required driving force value is equal to or less than a first threshold value, the driving force is generated only by the motor/generator, and when the driver required driving force value becomes more than the first threshold value, the engine is started. In this case, a threshold value for stopping the engine in order to prevent hunting is set to a second threshold value less than the first threshold value.
When an accelerator pedal is suddenly depressed under a state in which the accelerator pedal is released (under a regenerative braking state by the motor), as shown in FIG. 10, a driver required driving force value Fin before the filtering rapidly increases, and is then stabilized. In this example, the driver required driving force value Fin becomes more than a first threshold value ref1 in the course of the increase. On the other hand, a driver required driving force value Fout after the filtering transitions so as to follow the driver required driving force value Fin to increase halfway, but in the vicinity after the first threshold value ref1, turn to decrease to become less than the second threshold value ref2, and then turn to increase to approach the driver required driving force value Fin. Thus, the control state of the engine switches alternately in such a sequence as stop→start→stop→start. In other words, each time the driver required driving force value Fout passes across the dead zone, the control of the engine presents hunting. When this phenomenon occurs, the driver may feel a sense of discomfort.
Moreover, for example, as shown in FIG. 11, the driver required driving force value Fout after the filtering may temporarily become much more than the driver required driving force value Fin before the filtering. FIG. 11 is a graph for showing a state in which the driver is depressing the accelerator pedal, then suddenly releases the accelerator pedal, and then soon depresses the accelerator pedal again. In this case, there is such a fear that the driver may feel an unintended sense of acceleration.
The digital filter is used for various control apparatuses as well as for damping control for the vehicle body. However, as described above, there is such a fear that the operation of the device may become unstable when a device is controlled based on the output signal value of the digital filter.